A single umbilical artery (SUA), resulting in a two-vessel cord instead of a three-vessel cord, is the most common umbilical cord abnormality in humans with a prevalence of 1 in 100–200 pregnancies. In approximately 65% of cases, SUA is an isolated finding. Of those cases with SUA, the left umbilical artery is absent in approximately 70%. The incidence of SUA in twins is higher than in singletons, with no relation to chorionicity. Approximately 33% of fetuses with SUA have additional structural anomalies, and 10% are affected by aneuploidy. The most common chromosomal abnormalities are trisomy 18, trisomy 13, and triploidy, which together account for 82.9% of cases. An SUA is associated with over twofold increased risk of intrauterine growth restriction (IUGR), threefold increased risk of renal anomalies, and >20-fold increased risk of cardiac anomalies. Abnormalities affecting cardiovascular, skeletal, gastrointestinal, genitourinary, and central nervous systems are found in 20% of cases. It is usually identified sonographically when an artery is visible coursing adjacent to the fetal bladder along only one side. Although SUA may be visible in the first trimester, it is usually identified during the routine second-trimester fetal anatomic survey. The prognosis of the fetus with an SUA is determined by the presence of other malformations; in isolated cases, the prognosis is normal.